1. Field
The present disclosure relates generally to comparators. More specifically the present disclosure related to a comparator for MP3 playback having dynamic peak bias current that automatically adjusts quickly according to the load current variations.
2. Background
In certain functional operations, such as MP3 playback, low standby current and playback current/power (e.g., 0.1 mW) are required. A buck/flyback converter requires a fast, low power comparator for inductor current control. Due to space and cost constraints in mobile applications, small size inductors are desirable. Small inductors have low saturation current, which forces the design to work with non-linear inductors, in which magnetic materials exhibit nonlinear permeability over a range, which makes the resulting inductance a function of the applied current. However, because the inductor value drops, the current increases very quickly while in magnetic saturation. To keep device reliability and limit input current, a fast peak current comparator is desired. Thus, the speed requirement of the comparator conflicts with the low power requirement.
MP3 playback may be incorporated in wireless communication devices. Wireless communication devices have become smaller and more powerful as well as more capable. Increasingly users rely on wireless communication devices for mobile phone use as well as email and Internet access. At the same time, devices have become smaller in size. Devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and other similar devices provide reliable service with expanded coverage areas. Such devices may be referred to as mobile stations, stations, access terminals, user terminals, subscriber units, user equipments, and similar terms. As an example, smart phones typically incorporate digital music files for playback, expanding the consumer entertainment value of the device. Smart phones are typically used as much for their entertainment value (e.g., games, music and video streaming and playback) as they are for communications.
Several examples of fast current comparators exist. For example, (referring to FIG. 1), class-H power amplifier solutions may use a buck/negative charge pump (NCP) to generate positive and negative supply envelope for the output signal of a power amplifier (PA). Due to prior knowledge of signal in the digital signal chain, the reference voltage for the buck regulator can be generated in advance. Referring to FIG. 2, the load current of the buck regulator is predictable because of the known value of the PA output signal and standard load resistance.
Buck efficiency is optimized by tightly controlling inductor current. However, buck inductors may add significant cost. A class-G switched capacitor (SC) power converter may be used, however, it can only generate discrete output levels and also because such switches may require significant die area. To save cost, ferrite beads may be used to replace inductors. However, ferrite beads have poor inductance vs. current (L vs. I) characteristics.
Therefore, there is a need in the art for a high speed comparator to control inductor current.